1. Cross Reference to Related Application
This application is related to a co-pending application entitled “Continuous domain in-plane switch liquid crystal display,” which is assigned to the same assignee as this application.
2. Field of the Invention
The present invention relates to IPS (in-plane switching) mode LCDs (liquid crystal displays), and particularly to an IPS LCD which provides continuous domains when in a driven state.
3. General Background
A liquid crystal display utilizes the optical and electrical anisotropy of liquid crystal molecules to produce an image. The liquid crystal molecules have a particular passive orientation when no voltage is applied thereto. However, in a driven state, the liquid crystal molecules change their orientation according to the strength and direction of the driving electric field. A polarization state of incident light changes when the light transmits through the liquid crystal molecules, due to the optical anisotropy of the liquid crystal molecules. The extent of the change depends on the orientation of the liquid crystal molecules. Thus, by properly controlling the driving electric field, an orientation of the liquid crystal molecules is changed and a desired image can be produced.
The first type of LCD developed was the TN (twisted nematic) mode LCD. Even though TN mode LCDs have been put into use in many applications, they have an inherent drawback that cannot be eliminated; namely, a very narrow viewing angle. By adding compensation films on TN mode LCDs, this problem can be ameliorated to some extent. However, the cost of the TN mode LCD is increased. Therefore, a totally different driving means called IPS (in-plane switching) was proposed as early as in 1974. Then in 1993, Hitachi Corporation filed its first U.S. patent application concerning IPS LCDs, in which a particular IPS mode LCD was disclosed.
A conventional IPS mode LCD has an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. The liquid crystal layer has a plurality of liquid crystal molecules which have a same orientation when not driven, this orientation being parallel to the substrates. Pixel electrodes and common electrodes are disposed on the lower substrate. When a voltage is applied to the electrodes, an electric field is generated between the electrodes. The electric field drives the liquid crystal molecules to rotate, so that they have a new orientation that is still parallel to the substrates. The change in orientation results in a change in light transmission. In other words, the operation of the IPS mode LCD is such that the liquid crystal molecules rotate in a plane parallel with the substrates in order to fulfill optical switching. The displayed image has the important advantage of a wide viewing angle. In basic IPS mode LCDs, the pixel electrodes and common electrodes are each comb-shaped. The electric field of these LCDs in a driven state is along a certain direction. When the displayed image is viewed at different oblique angles, an observer can notice a quite large color shift.
Referring to FIG. 5, this is a top cross-sectional view of a pixel area of an IPS liquid crystal display as disclosed in U.S. Pat. No. 6,459,465 issued on Oct. 1, 2002. The pixel area includes a gate line 113 arranged in a first direction, a data line 115 and a common line 135 both arranged in a second direction orthogonal to the first direction, a pixel electrode 131, a common electrode 133, and a TFT (thin film transistor) 120 positioned at an intersection of the data line 115 and the gate line 113. The TFT 120 has a gate electrode 121, a source electrode 123 and a drain electrode 125, which are connected with the gate line 113, the data line 115 and the pixel electrode 131 respectively. The pixel electrode 131 and the common electrode 133 are spaced apart from each other. The pixel and common electrodes 131, 133 are each generally comb-shaped, with the teeth thereof being zigzagged. First portions of the teeth of the pixel and common electrodes 131, 133 that are aligned in a first direction and are parallel to each other form a first sub-electrode group. Second portions of the teeth of the pixel and common electrodes 131, 133 that are aligned in a second direction and are parallel to each other form a second sub-electrode group.
When a voltage is applied, because the pixel and common electrodes 131, 133 have the zigzagged structures, the electric field (not shown) generated is mainly along two directions. Referring to FIG. 6, the upper portion thereof shows part of the first sub-electrode group, and the lower portion thereof shows part of the second sub-electrode group. The liquid crystal molecules 130 in the upper and lower portions have different orientations, and the LCD exhibits a two-domain display effect. When viewing the LCD display from any oblique angle, the color shifts generated by the two domains counteract each other, and thus the overall color shift of the display is small.
However, the two-domain electrode configuration of the LCD inherently limits the display thereof. Equally good visual performance at various different viewing angles cannot generally be attained.
What is needed is a multi-domain IPS liquid crystal display which has a high contrast ratio and improved wide viewing angle characteristics.